Since its invention, the bipolar junction transistor (BJT) has had an enormous impact on virtual every area of modern life. Since this time, researchers have been working diligently to ever expand the limits of its performance. For example, much effort have been focused on the fabrication of high frequency transistors having extremely small device sizes. In such devices, it is important to keep the emitter, base, and collector resistances at a minimum when considering the design of a high frequency device since the effective resistance associated with each area affects the various RC charging times.
Consider the doping profile of a typical prior art bipolar junction transistor as shown in FIG. 1. As can be seen, the emitter doping concentration is extremely high--on the order of 10.sup.20 -10.sup.21 atoms/cm.sup.2. The doping concentration in the base region typically ranges from 10.sup.17 14 10.sup.18 atoms/cm.sup.2. The collector region is normally the most lightly-doped region of the bipolar junction transistor.
As shown in FIG. 1, the collector region usually comprises an n-type epitaxial layer which is doped to approximately 10.sup.15 atoms/cm.sup.2. It is worth noting that the collector doping profile is intentionally kept flat from the base collector junction (e.g., point A) extending down to the more heavily doped n+ buried layer region (e.g., beginning at point B). Minimizing base-collector capacitance is one of the chief reasons why the doping level in the epitaxial collector region is kept at a minimum.
One of the common problems associated with the standard bipolar junction transistor is that the performance of the device is limited at high current densities. This limitation arises because of the so-called "Kirk Effect". The Kirk Effect--also frequently referred to as current-induced base push-out--occurs at very high injection levels and very high current densities. In modern BJTs having a lightly-doped epitaxial collection region, the current gain is directly affected by the relocation of the high-field region from point A to point B (see FIG. 1) under high-current conditions. This high field phenomena (i.e., the Kirk Effect) effectively increases the base width from W.sub.B to (W.sub.B +W.sub.C). Increasing the effective base width in this manner also increases the effective base Gummel number, which represents the number of impurities per unit area in the base region. A higher Gummel number causes a corresponding reduction of the current gain in the device.
The Kirk Effect also influences the high frequency performance of bipolar transistors. Because of the Kirk Effect, there is an optimum collector current that gives the maximum cut-off frequency for the standard BJT. This behavior is shown by the solid line in FIG. 2. Beyond the optimum collector current level, further increases in current are made possible only by lowering the maximum frequency of operation of the device (i.e., f.sub.T).
One prior art approach for combating the adverse effects of the Kirk Effect involves uniformly increasing the doping throughout the epitaxial collector region. The problem with this approach is that increasing the collector doping throughout the epitaxial layer greatly increases the collector junction capacitance, resulting in a lower f.sub.T. This latter approach is shown by the dashed line in FIG. 2. Thus, a higher epitaxial doping level permits a moderate increase in the current handling capacity of the device, but only at the expense of lowered frequency performance.
Therefore, what is needed is a bipolar junction transistor capable of suppressing the Kirk Effect to allow much higher current densities than achievable under prior art approaches.